1. Field of the Invention
The present invention relates to a control apparatus for a charging generator for charging a battery by controlling the output voltage of the generator in a predetermined value.
2. Description of the Prior Arts
FIG. 1 shows a diagram of the conventional apparatus of this type and FIGS. 2, 3 and 4 show characteristic curves of the apparatus. In FIG. 1, an alternating current generator (1) equipped in a vehicle (not shown) and actuated by an internal combustion engine (not shown) has armature coils (101) connected having the three phase star connection and a field coil (102). A full-wave rectifying means (2) is provided to rectify the AC output of the generator (1) and has a rectified current output terminal (201) at the positive side and a rectified current output terminal (202) at the negative side. A voltage regulator (3) for controlling the output voltage of the generator (1) in a predetermined value comprises a surge absorbing diode (301) connected at both ends of the field coil (102); output transistors (302), (303) connected each other into the Darlington connection so as to turn-on or off the field current of the field coil (102); a resistor (304) connected in the base circuit of the transistors (302), (303); a control transistor (305) for turning on or off the output transistors (302), (303); a Zener diode (306) which detects the output voltage of the generator (1) to turn-on when the output voltage reaches a predetermined value; diodes for detecting temperature (307), (308), (309), (310) which are connected in series in the forward direction to the Zener diode (306) to give a negative temperature gradient for the predetermined value; resistors (311), (312) connected in series between the rectified current output terminal (201) at the positive side and the rectified current output terminal (202) at the negative side to form a shunt circuit for voltage; a battery (4) and a key switch (5).
The operation of the conventional apparatus having the structure will be described.
When the key switch (5) is closed to actuate the internal combustion engine, the base current is fed from the battery (4) through the key switch (5) and the resistor (304) to the output transistors (302), (303) to turn-on the latters. Accordingly, the field current is fed from the battery through the key switch (5) to the field coil (102) to produce the field magnetomotive force. When the engine is started to drive the generator (1) in this condition, an AC output is induced in the armature coil (101) depending on the revolution speed of the generator. The AC output is subject to full-wave rectification by the full-wave rectifying means (2). When the voltage of the rectified output is lower than the predetermined value, the Zener diode (306) holds the turn-off condition since the potential at the shunt point of the shunt circuit comprising the resistors (311), (312) is sufficiently low so that the output voltage of the generator (1) increases depending upon the increase of the revolution speed. When the revolution speed of the generator (1) further increases and the output voltage exceeds the predetermined value, the potential at the shunt point of the shunt circuit is also high whereby the Zener diode (306) is turned on to feed the base current to the control transistor (305) through the diodes (307), (308), (309), (310) and the Zener diode (306) thereby turning on the control transistor (305). When the control transistor (305) is turned on, the output transistors (302), (303) are turned off to de-energize the field coil (102) so that the output voltage of the generator (1) decreases.
When the output voltage decreases lower than the predetermined value, the Zener diode (306) and the transistor (305) are turned off again and the transistors (302), (303) are turned on to actuate the field coil (102) so that the output voltage of the generator (1) increases again. The operation described above is repeated to control the output voltage of the generator (1) in the predetermined value. Thus the battery (4) is charged in the predetermined value by the controlled voltage.
FIG. 2 shows a static characteristic curve depicting the variation of a controlled voltage to that of the ambient temperature of the voltage regulator (3) and the generator (1). As shown in the figure, the controlled voltage tends to decrease linearly depending upon the increase of the ambient temperature because the rate of the voltage drop in the forward direction of the temperature detecting diodes (307), (308), (309), (310) is respectively -2 mv/.degree.C.
FIG. 3 shows a dynamic characteristic curve depicting the variation of the controlled voltage to the operating time of the conventional apparatus shown in FIG. 1, that is, the variation of the controlled voltage to the operating time when the generator (1) is operated at a constant revolution speed under the condition passing load current by the connecting of a load (not shown). As shown in the figure, the self-heating occurs in the generator (1) and the voltage regulator (3) since the field current increases in substantially proportional to the load current. Usually, the temperature detecting diodes (307), (308), (309), (310) and the output transistors (302), (303) are placed in a position subject to the affection of heat and the self-heating of the generator (1) affects to the voltage regulator (3) through the heat conduction. Accordingly, the temperature of the temperature detecting diodes (310), (308), (309), (310) increase depending upon time and the controlled voltage is decreased by .DELTA.V.sub.1 to be saturated in accordance with the characteristic curve shown in FIG. 3.
FIG. 4 shows an external load characteristic curve depicting the variation of the controlled voltage to that of the load current wherein the controlled voltage is usually decreased by .DELTA.V.sub.2 depending upon the increasing of the load current.
In the conventional apparatus having the static characteristic shown in FIG. 2, when the generator is operated under a heavy load, the controlled voltage decreases by .DELTA.V.sub.1 regardless of the ambient temperature as shown in FIG. 3. This is disadvantageous in charging the battery (4). Furthermore, when there are many loads, the controlled voltage decreases by .DELTA.V.sub.2 as shown in FIG. 4 and after all, the controlled voltage decreases by .DELTA.V.sub.1 +.DELTA.V.sub.2 whereby the charging characteristic of the battery (4) becomes inferior.